1. Field of the Invention
The present invention relates to a method for computing the coordinates of images of marks in a volume dataset, the coordinates referencing the actual position and orientation coordinates of a physical object or living being. The invention further relates to a medical device with a data processing device for storing a volume dataset and a navigation system.
2. Description of the Prior Art
A method and medical device of the above type are needed in the navigation of a medical instrument or in image fusion. An example of navigation of a medical instrument is described in German OS 199 51 502. A volume dataset of the body region of interest of a patient is generated by means of a computed tomography device. During a minimally invasive intervention, in particular, a surgeon guides the medical instrument into the body of the patient. A position sensor of the navigation system is disposed at the medical instrument, so that the navigation system determines the position, i.e. the location and orientation, of the medical instrument relative to the patient's body. Based on the position determination, the determination of the location coordinates of the medical instrument, an image of the medical instrument can then be merged into the image that is allocated to the volume dataset, which is displayed on a monitor.
The volume dataset can be produced preoperatively, e.g. with a computed tomography apparatus. Fading in the image of the medical instrument requires a spatial transformation of the coordinates of the position sensor of the navigation system, which is disposed at the medical instrument in a defined manner, the coordinates referencing a first coordinate system, into the spatial coordinates of the image of the patient that is acquired with the computed tomography apparatus and utilized for the navigation. This transformation is referred to as registration.
Marks are made on the patient, usually on the body surface, for registration. Locations in the environment of the operation area that move as little as possible are selected. The marks remain fixed on the patient during the entire image supported procedure. i.e. both during the recording of the volume dataset and during the intervention. The contrast of the marks can be relatively weak, because they are frequently relatively small for reasons of space. Furthermore, relatively weakly contrasting marks are used so that the remaining image volume is impaired as little as possible by artifacts. Given the quality of modern computed tomography or magnetic resonance devices, it is usually no problem to tap these marks manually, for instance in two orthogonal views, in order to determine their spatial positions. This technique is relatively time-consuming, because it is usually necessary to sift through almost the entire volume dataset in order to find the images of the marks in the volume dataset. Moreover, at least three images of marks must be captured.
In U.S. Pat. Nos. 5,769,789, 5,799,099 and 6,052,477, a method is described in which the search, for instance for MR and CT head shots, is performed automatically. The entire volume dataset is examined during the search. Substantially morphological operators are applied, which correspond to the mark dimensions, and a threshold separation is performed. This procedure is relatively calculation-intensive. Furthermore, the application of the filter operations to the entire volume dataset raises the probability of falsely recognized images of marks. The method is also limited in the specificity of the operators, because markers are oriented randomly in space.
Each image of a mark that is identified in the volume dataset must be tapped on the patient with the position sensor of the navigation system for the actual registration. This produces the relation of the coordinates of the volume dataset to the coordinate system of the navigation system.
A corresponding transformation matrix is calculated from the coordinates of the marks and the coordinates of the images of the marks in the volume dataset, and the registration is terminated.
Another application in which the coordinates of images in a volume dataset are needed is when a corresponding CT view is needed, particularly for a current endoscopic, laparoscopic, or ultrasound image, in order to superimpose the two views.